Fiber Connector End Faces & Insertion Loss – Luna Innovations

Why am I experiencing high insertion loss?

Most high insertion loss traces back to the connector end face: a dirty, damaged or mismatched connection. Inspect and clean every connector before you mate it, and most cases resolve without a service request.

Insertion loss (IL) is the optical power lost where two fibers meet, measured in dB. A clean, well-mated connector pair adds only a fraction of a dB; anything that blocks, scatters or misdirects light at the joint raises it. This article lists the common causes, the inspect-clean-inspect procedure Luna uses, and when to contact Luna Technical Support.

Common causes

Contamination is the most frequent cause, and the easiest to fix. Work down this table in order.

Cause	What happens	What to do
Contamination (dirt, dust, oils)	Even tiny particles on the core block or scatter light. Dirt also transfers between mated connectors.	Inspect both sides; clean and re-inspect (see below).
End-face damage or scratches	Pits, chips or scratches in the core zone scatter light. Often caused by mating dirty connectors.	Re-inspect after cleaning; replace or re-terminate the jumper if damage remains in the core.
Misalignment of fibers	Cores that don't line up (poor installation, worn sleeve, damaged fiber) pass less light.	Re-seat the connector fully; for FC, finger-tighten the coupling nut and align the key. Try a known-good jumper.
Improper polishing	An irregular end-face geometry prevents good physical contact.	Replace the jumper with a factory-polished, tested one.
Air gaps	A small gap between ferrules reflects and scatters light.	Make sure the connector is fully seated and latched; check for debris holding the ferrules apart.
Connector type mismatch	Mixing polish types (PC/UPC with APC) leaves a gap and an angle mismatch.	Match polish types on both sides (see below).

Inspect, clean, re-inspect

Inspect before every connection, clean only when needed, and never mate a connector that hasn't passed inspection.

Tools Luna uses

Inspection: EXFO FIP-430B automated inspection probe, which gives a pass/fail result and a saved image.
Acceptance criteria: IEC 61300-3-35 Ed. 3, Singlemode RL ≥ 26 dB.
Cleaning: Jonard Tools FCC-250 click-to-clean cleaner (demonstration video). Fiber ports follow international standards, so the cleaner works the same on any instrument brand.

Procedure

Turn off or disable the laser source before inspecting. Never look into a fiber or port.
Inspect the instrument port and the cable connector. Record the result.
If either fails, clean it:
- Instrument port (2.5 mm bulkhead): remove the blue dust cap from the cleaner tip, insert the tip straight into the port, and push forward firmly until you hear a click.
- Cable connector: keep the blue dust cap on, flip open the small tab at the end of the cap, insert the connector ferrule, and push until it clicks.
Re-inspect. Repeat the cleaning up to two or three times.
If it still fails, look for scratches or pits in the core zone. Damage that cleaning can't remove means the jumper or port needs replacement or service.
Mate the connectors and re-measure insertion loss.
Cap every unused port and connector right away.

Connector polish types: PC, UPC, APC

Only mate APC to APC, and PC/UPC to PC/UPC. Mating APC with a flat-polish connector leaves an angled air gap that causes high loss and can damage both end faces.

Polish	End face	Usual color	Mates with
PC	Slightly domed, flat contact	Blue or black	PC, UPC
UPC	Finer domed polish, lower back reflection	Blue	PC, UPC
APC	Polished at an 8° angle	Green	APC only

Check the instrument's part number for its port type. For example, a model ending in "FC/APC" has FC connectors with APC polish, so use FC/APC jumpers.

Best practices

Inspect both sides of every connection, every time, including new jumpers straight from the bag.
Keep dust caps on all ports and connectors when not in use; caps themselves can carry dust, so inspect after removing one.
Never touch the ferrule end face.
Use a dedicated fiber cleaner, not compressed air or shirt cloth.
Don't over-tighten FC coupling nuts; finger-tight is enough.
Replace worn or frequently mated jumpers instead of the instrument's port; use a sacrificial patch cord where possible.

Still seeing high loss?

If a connector still fails inspection after cleaning, or insertion loss stays high with known-good jumpers, contact Luna Technical Support.

Submit a request through the Luna Support Portal (how to enter a support ticket).
Include the instrument model and serial number, the measured insertion loss, and inspection images or pass/fail results for each connector.
Support will troubleshoot with you and, if needed, start an RMA.